I would suggest a a 4-6 year tenure for people coming to the end of a long career devoted to service. The sorts of people who are awarded OBEs/MBEs (yes I realise the irony but I'm working with what we've got), or who become the heads of Oxbridge Colleges, or who have simply had a long, quiet career of service. The sorts of people who have served deprived communities, or in the worlds of international relations, law, diplomacy, charities, academia and teaching, medicine, the arts, sports. Noone outside the world of medicine had heard of Chris Whitty before the current crisis, but what a remarkable job he has done of straddling the worlds of medicine and science, and politics, with dignity and diplomacy.
Not people who have had millions thrown at their education and lifestyle and then go on Oprah, hang out with paedophiles and trafficked girls, or at best, dip a toe now and again into others lives before retreating to their lavish mansions.
Seems a no brainer to me. Obviously some will be more successful than others but the current situation is unsustainable. There could be a shortlist, drawn up by the sorts of people who recruit the heads of colleges, which I think is a similar ceremonial role that attracts a similar late career person. A role in which personal characteristics, dignity and service are important. Some heads of college require a lot of management behind the scenes, but the benefit of this role would be the short tenure; heads of college are hard to get rid of! And there are many remarkable, dignified, inspiring people who could bring such a great deal to this sort of role in public life.